High frequency resistance is a well-known and extensively documented property of fuel cells. By measuring the high frequency resistance of a fuel cell within a specific band of excitation current frequencies, the state-of-humidification of a proton exchange membrane can be measured. The high frequency resistance in a proton exchange membrane of the fuel cell is typically measured at a single frequency between 500 Hz and 1500 Hz.
Traditional methods for measuring high frequency resistance begin by first inducing an alternating current at the frequency of interest (typically 1000 Hz) through the fuel cell or stack. Next, the instrument measures the actual current ripple flowing through the fuel cell or stack and the voltage ripple induced on the cell or stack via the injected alternating current. These signals are filtered and amplified by the instrument. The instrument then determines the high frequency resistance by dividing the magnitude of the voltage ripple waveform by the magnitude of the current ripple waveform. In accordance with Ohm's law, the resulting value is a resistance, which may be further scaled by the number of cells in the stack or the active area of the fuel cell membrane to yield the unit-area-resistance of the membrane.
In noisy electrical environments, such as that of a high voltage distribution system in an electrically-propelled vehicle, other large alternating current ripples may be present in addition to the induced ripple current and resultant ripple voltage. These large undesirable signals may exist at a single frequency persistently, or they may have variable frequency content depending on vehicle speed, engine load, or other factors.
The presence of such large undesirable signals can be problematic for traditional single-frequency high frequency resistance measurement systems. For example, if the induced ripple current is approximately 0.3 A, and the large undesirable signal is 100 A at the same frequency, a circuit with a dynamic range greater than 70 db must be realized to prevent saturation of the signal chain.
It would be desirable to produce a high frequency resistance measurement instrument with a reasonable dynamic range, which militates against saturation in the presence of one or more large undesirable signals. It would be further desirable to produce a high frequency resistance instrument that can be employed at multiple induced AC currents of differing frequencies to maintain measurement capability despite signal chain saturation in the presence of a large undesirable signal.